Transmissive LCD devices use internally supplied illumination. In a traditional illumination system, one or more light sources, e.g., incandescent or florescent lamps, are placed behind the display, i.e., backlighting the display.
Backlighting LCDs with lamps has several problems. For example, the incandescent lamps can lack uniformity in illumination from localized “hot spots” which reduce the display's readability. In addition, incandescent lamps can be costly to operate. Florescent lamps help to control the energy costs, however, these lamps are subject to the same unevenness in illumination as incandescent lamps.
The illumination or intensity of the light in the backlighting system should remain constant. In the case of lamp systems, a light sensing device is often placed inside the backlighting cavity to detect the systems illumination. In bright light settings, the sensor inside the cavity can falsely perceive some of the light and cause the lamp to reduce luminescence. This effect is opposite of what is desired in this setting. To accurately view the display in bright light, the display should be brighter, not dimmer.
One attempt to overcome the problems with ambient light, involves placing the light sensing device outside the backlight cavity. The length of the fluorescent lamp legs are extended beyond the cavity so luminance can be extraneously monitored. While this sensing technique may reduce the effects of ambient light, the efficiency of the lamp declines because some of the available light is not available to the display. Moreover, placing sensors near the ends of the lamp legs brings them in close proximity to the lamp filaments. The filaments produce a constant infrared radiation which is easily read by the sensors and thus falsifies the “true” luminance of the lamp.
Many displays do not provide for intensity control simply because of the inaccuracies in the previous illumination systems, cost, and size concerns. For instance, laptop computers, cellular telephones, and handheld computing devices typically do not include an intensity control mechanism for the display. The user becomes a slave to the devices external environment. In other words, a visible display indoors is often too dim to be viewed outdoors. Users are forced to find alternative methods to view the display, such as holding their hand over the display to block light.
In some devices, display illumination can be manually controlled. For example, when a moving vehicle enters a tunnel, the operator may be able to manually adjust the intensity of the dashboard lights simply by turning a control. In some situations manually controlling illumination may be an unacceptable task. For instance, in an airplane cockpit there are multiple displays which must be viewable in all lighting conditions. The pilot cannot be bothered with manually setting the display brightness for such highly sensitive information.
Recently, LCD supplemental illumination techniques using light emitting diodes (LEDs) have been introduced. This approach provides a whole array of LEDs instead of one lamp. There are side lighting configurations of LEDs, see e.g., U.S. Pat. No. 5,046,829, as well as “direct view” attempts, see e.g., U.S. patent application Ser. No. 09/384,137 which is incorporated herein by reference. While many of the problems associated with lamp backlighting techniques are eliminated with LED backlightling systems, maintaining constant illumination and providing an accurate luminance sensing technique remains a problem. The '137 application identifies the need for adjusting the illumination intensity of the LED array; however, the large quantity of LEDs employed and the number of unique light sources presents a significant challenge in obtaining accurate light sensing.
Ideally, the light emitted from an LED can be sensed as it is perceived by the viewer. However, placing a sensing device in the direct view of the light would block the light to the display and create a “dark spot” on the display. Alternatively, a sensing device could be mounted on a side panel inside the cavity to avoid blocking the emitted light. This approach is prone to erroneous false readings from direct light similar to the ambient problems previously discussed.
Thus, a luminance sensing technique for LEDs is needed. Moreover, a luminance sensing technique for a desired LED backlit LCD which is reliable and able to maintain a viewable display even under changing ambient lighting conditions in needed.